1. Field of the Invention
The present invention relates to a two-cycle combustion engine of an air scavenging type which may be used as a drive source for a portable work machine such as a brush cutter, and, more particularly to a cylinder block for such engine and a method for manufacturing such engine cylinder block.
2. Description of the Prior Art
It is well known that some of the two-cycle combustion engines currently available in the market employ an engine cylinder block of a type, in which a scavenging passage defining wall is provided in a region confronting the cylinder bore so as to define a part of the scavenging passage communicating between a combustion chamber and a crankcase chamber. When this type of cylinder block is formed with the use of a molding die, a scavenging port defined above the scavenging passage defining wall is in the form as undercut. Accordingly, the Japanese Laid-open Patent Publication No. 2000-145536 discloses the use of, for example, a disposable core for defining the scavenging port when the cylinder block of the above discussed type is to be formed. On the other hand, the Japanese Patent Publication No. 58-31461 discloses the use of a slider core capable of being slid in a direction radially of the cylinder block in an inner mold for defining the cylinder bore in the cylinder block.
However, where the disposable core is used, it is necessary for the core to be set in the mold assembly each time the cylinder block is formed and, therefore, the workability is low. Furthermore, to manufacture a number of cylinder blocks, a corresponding number of disposable core are required, resulting in increase of the manufacturing cost. Inconveniences are also experienced in association with disposal of the disposable cores, which may cause a problem of industrial waste treatment. On the other hand, the use of the slidable core requires the use of a complicated mold assembly and, also, since the slidable core tends to be worn quickly, a frequent replacement of the slidable core is required, resulting in reduction of the workability and the productivity along with an increase of the manufacturing cost.